WASHINGTON (Reuters) - A series of mock terrorist attacks began on Saturday in the United States, testing the ability of top local, state and federal officials to respond to a catastrophic sequence of biological and chemical releases.

In Portsmouth, New Hampshire, a spoof explosion of a Chevrolet van loaded with unknown chemical agents kicked off the drill a few hours after daybreak near the town's port, fictionally killing and maiming around 50 people.

A biological "attack" took place outside Denver, Colorado, and a third series of exercises were being run in and around the nation's capital, the Justice Department said.

Called "Topoff" -- short for Top Officials -- the largest ever such dress rehearsal is being conducted by the Department of Justice and the Federal Emergency Management Agency.

Thousands will participate in the 10-day exercise, including Attorney General Janet Reno, other Cabinet members, mayors, local and state police, hospital personnel and volunteer actors playing the role of injured and dead civilians, according to the Justice Department.

"The goal of the exercise is to assess the nation's crisis and consequence management capacity under extraordinarily stressful conditions," said Gina Talamona, a Justice Department spokeswoman.

At a cost of $3.5 million, Topoff is the largest drill of its kind ever conducted in the United States, and while the various officials knew the exercise was to take place, they did not know before the exercise the size and scope of what they were going to have to deal with, and still may not.

"No one was surprised, that's not the critical part. People knew this was a drill they were responding to, but not to what they were responding to," said Jim Van Dongen from the New Hampshire Office of Emergency Management.

NO PLANS FOR PUBLIC PANIC

"Topoff" is being carefully orchestrated so as not to alarm the general public. Police in responding areas are not using sirens and though physical activity is taking place on the ground, it is not being done in a way to excite the uninformed.

"The intent is to make sure this does not turn into the War of the Worlds," said a source with the Clinton administration, referring to the radio play produced by Orson Welles in the 1930s which scared many Americans into thinking Martians were attacking the planet.

In the Portsmouth attack, local officials said shortly after 8:20 a.m. EDT, at the start of a make believe charity foot race, a fake car bombing sent a foul smelling concoction of garlic mixed with Gatorade over the picturesque Portsmouth Port Authority facility.

"A Portsmouth police officer was the first on the scene, then fire trucks," said Van Dongen.

Following procedure, local authorities called in toxic hazard teams, then informed the statehouse of what was happening. Federal involvement followed, after the governor's office requested a presidential disaster declaration, freeing the FBI and FEMA to respond to the incident.

FBI Special Agent Barry Mawn told reporters the exercise was "going pretty well," as federal agents and police scurried around the dead and wounded in Portsmouth.

The spoof terror incident in Colorado actually began two days ago under a scenario in which a "terrorist" released "anthrax spores" in Denver.

Police were called to a Denver hotel room after a body was found. But what first looked like a simple death soon turned into something else when police noticed vomit and blood.

Other "information" that the Federal Bureau of Investigation had "received" about the same time prompted sending hazardous material personnel to the scene.

Soon the bomb squad was dispatched to the hotel room, which was actually on an old army hospital. Other events were expected during the 10-day exercise in the Denver metropolitan area.

CONGRESS MANDATED TERROR DRILLS

"Topoff" stems from a provision inserted in a 1998 spending bill by Republican Sen. Judd Gregg of New Hampshire calling for "practice operations" for a terror attack.

Critics of the exercise say it overstates the real threats facing America and that such a broad range of attacks would be unlikely to happen all at the same time.

In congressional testimony in March, CIA Director George Tenet said major threats to U.S. security came from groups such as Saudi exile Osama bin Laden's, which were trying to acquire biological and chemical weapons capabilities.

Tenet said over the next few years U.S. cities faced ballistic missile threats from a wide variety of sources: North Korea, probably Iran and possibly Iraq.

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